The invention relates to an aqueous dispersion of modified polyurethane resins which is suitable for the production of coating agents that are capable of being diluted with water, in particular for the generation of base layers in multilayer top coatings. It relates also to the production of the dispersion of the polyurethane resins which contain polymerised unsaturated monomers, and also to the use thereof. The invention also relates to polyurethane macromonomers which can be used in order to produce the modified polyurethane resins.
In particular in the lacquering of automobiles but also in other fields in which coatings having a good decorative effect and at the same time good protection against corrosion are desired it is known to provide substrates with several coating layers disposed above one another. In this connection an undercoat, a corrosion-protection layer for example, is generally firstly applied, then optionally intermediate layers, and then a coloured coating agent is also applied, followed by a transparent clear lacquer. In the coloured base layer use is made of metallic pigments or coloured pigments which result in metallic lacquer coatings or unilacquer coatings. Use may be made in this connection of coating agents which crosslink at elevated temperatures, or possibly of coating agents which dry or crosslink at temperatures below 100.degree. C.
In the lacquering of automobiles use has previously been made in this connection of coating agents with a high content of organic solvent. With a view to reducing environmental pollution, efforts are being made to avoid using organic solvents in coating agents as far as possible.
Coating agents are known which predominantly contain water as solvent or dispersing agent and which contain organic solvents only in small amounts. The use of polyurethane dispersions (PU dispersions) in base lacquers is also known. For instance, in EP-A-0 089 497 metallic base lacquers are described which contain aqueous PU dispersions as binding agents. These binding agents contain ionic groups in the molecule. They are formed by synthesis of a prepolymer containing NCO groups which in aqueous dismersion is subjected to chain extension with diamines.
In DE-OS 26 63 307 and in DE-OS 19 53 348 the emulsion polymerisation is described of vinyl monomers in polyurethane dispersions based on OH-containing polyesters comprising ionic groups, whereby said polyesters serve to produce films. The vinyl monomers are acrylate esters or styrene, which may also contain functional groups.
In U.S. Pat. No. 4,318,833, with a view to producing metallic coating lacquers, aqueous dispersions of polyurethanes of low molecular weight containing carboxyl groups are reacted with ethylenically unsaturated monomers, whereby said dispersions contain large amounts of solvent. The lacquers which are formed exhibit inadequate resistance to water. In addition, the amounts of organic solvents are undesirable.
In the production of the PU dispersions described in the above references to the literature the chain extension of prepolymers is effected exclusively via the formation of C--N or C--O bonds. No CH acidic groups or olefinic groups are present.
In DE-A-4 122 266 polyurethane dispersions with terminal double bonds are described which can be polymerised with unsaturated monomers. In this connection he double bonds are incorporated by reaction via OH--, SH-- or NH-functional unsaturated monomers into PU prepolymers terminated by NCO groups. Chain extension in the aqueous phase is not carried out.
Known from EP-A-0 367 051 are aqueous PU dispersions which comprise carbonyl groups that have been incorporated by reaction and also carbonyl groups. These functionalised polyurethanes are used as an aqueous dispersion. Crosslinking with polyhydrazides takes place in the dried film via the carbonyl group subject to formation of a C--N bond.
In the as yet unpublished DE patent application 42 28 851 made by the same applicant there are described aqueous PU dispersions in which chain extension of the structural units is carried out by reaction of CH acidic groups. But in neither reference are any further modifications to the PU polymers described.
The known aqueous coating agents which contain PU dispersions do not satisfy all stringent requirements. They have disadvantages in the adhesion to the additional layers of multilayer lacquer coatings. Problems arise in particular in the resistance to condensed moisture--i.e. when placed under load at elevated temperature and atmospheric moisture a peeling or blistering is to be observed. A further disadvantage of such dispersions is the amount of organic solvent that they contain. Furthermore the compatibility with additional binding agents in the coating agent is frequently problematic, so that the stability in storage is thereby reduced.